Photoreactivation of thymine dimers in UV-irradiated human cells: unique dependence on culture conditions.

UV-irradiated human fibroblasts in tissue culture were exposed to photoreactivating light in an attempt to demonstrate a light-dependent loss of thymine dimers from the acid-insoluble fraction of the DNA. The only experimental conditions in which this phenomenon was observed was if the cells were grown for at least 10 days in Dulbecco's modified Eagle's minimum essential medium. Such cells lost a maximum of between 10-30% of the thymine dimers from their DNA during illumination for 1 h. When cells were grown in a variety of other media the phenomenon was not observed. The present experiments do not discriminate between true enzymatic photoreactivation and a medium dependent photosensitization phenomenon that is not enzymatic in nature.